The properties of a reconstituted system of bovine cardiac contractile proteins are being studied as a function of the Ca2+ concentration. The actin-troponin-tropomyosin complex facilitates a cooperative Ca2+-induced activation of the MgATPase of cardiac myosin subfragment-1, under conditions where myosin subfragment-1 has been carefully excluded as a source of cooperativity. The details of this MgATPase activation also depend upon which one of two troponin T isoforms is present. These isoforms are known to differ near the amino terminus and are regulated in other species by alternative mRNA splicing.